Known methods for throttling the power output of a multi-stage steam turbine system use a divided steam feed system in which the steam enters the turbine inlet via numerous isolatable and individually controllable arcs of admission. In this method, known as partial arc admission, a number of active first stage nozzles is varied in response to load changes. However, partial arc admission systems in the past have been known to possess limited efficiency of work output across the control stage. Some of these limitations are due to unavoidable mechanical constraints, such as, for example, an unavoidable amount of windage and turbulence, which can occur as rotating blades pass nozzle groups that are not discharging steam. This can result in mechanical excitation of the blades, which can particularly impact the first blade rows that follow the control stage. To address this, the distance between the arcs of admission and the rotating blades can be increased to increase a volume for mixing and provide more even flow distribution to the blades. However this configuration increases overall turbine length.
It is also known to reduce the effect of mechanical excitation of airfoils and enable a shortening of the mixing section of a turbine by making blades and nozzles stiffer. However, such an approach is contradictory to the demand of increased efficiency as stiffer blades can reduce performance.
U.S. Pat. No. 4,780,057 A1 discloses a partial arc admission system having suitably arranged control stage nozzles with a variable aspect ratio wherein the variable aspect ratio can address steam distribution. U.S. Pat. No. 5,080,558 A1 discloses utilizing variably dimensioned control nozzles.